1. Field of the Invention
The present invention relates generally to improved sealing in fluid carrying devices such as double block and bleed plug valves and more specifically to an improved sealing configuration wherein elastomeric, secondary and tertiary seals supplement the stuffing box/packing gland sealing integrity and provide an over-pressure chamber for preventing environmentally unsafe leaks.
2. Prior Art
The environmental protection agency and other agencies are presently pressing the valve industry for improvements in stuffing box/packing gland integrity. Packing glands and similar mechanical seals, wear and age and eventually leak to atmosphere, creating environmental problems, especially when the media flowing through the valve is hazardous or life threatening. Clearly, all seals eventually age and wear and ultimately do leak. Accordingly, a dependable and provable method to predict leakage before it occurs, would be a highly desirable improvement in such applications.
Various methods are presently used to improve packing gland sealability and to prevent potential leaks therethrough. The method currently perceived to be the most dependable for improving packing gland sealability and predicting and controlling packing gland leakage, is known as a bellows seal. Bellows seals are simply flexible pressure vessels that are used in plug type valves in combination with the mechanical stuffing box/packing gland primary seals. However, bellows are limited in their axial and rotational capabilities. Furthermore, as they age, they are subject to fatigue and failure, consequently a monitoring or sniffing access is often required. An older method for improving packing gland sealability involves installation of two seals separated by a void that is usually filled with a device called a lantern ring. A cross drilled access hole is usually provided to monitor the void between the two seals. A "sniffer" may be used to monitor the leakage of the seal closest to the pressure source (primary seal) while the redundant seal (secondary seal) prevents interim leakage until the primary seal can be serviced. Unfortunately, merely providing a secondary or redundant seal to create a void between the primary seal and the redundant seal, allows accumulated, insignificant leakage to create false alarms, i.e., apparent but false indication of significant leakage of the primary seal which requires unnecessary and high cost maintenance.
There is therefore an ongoing need for a dependable method for improving the ability to predict leakage before it actually occurs in a fluid carrying device such as a plug type valve, in applications involving particularly hazardous or life threatening media flowing through the valve. Such a need includes improving the currently available sealing techniques and overcoming the fatigue and failure concerns regarding bellows and the frequent false alarms occurring in the use of a single secondary seal to create a void between the primary seal and the secondary seal.